Argument from ignorance:

When it is sugggested that something is true because it has not been proven false, or false because it has not been proven to be true.

Fairies must exist since we’ve seen no definitive evidence falsifying their existence.

Also known as: appeal to ignorance

Notes

Phrases that often accompany this fallacy include “What are the odds!” and “Explain that!” The inability of a human to grasp the stochastic or conceptual complexities of a particular concept do not warrant the introduction of magic or a god or luck as an explanation.
Note that there is warrant to dismiss out-of-hand claims that are logically or physically impossible. If I claim I possess a square triangle, or a flying reindeer, you are warranted in stating unequivocally that I am wrong.

Case Study One

Some say we are warranted in believing that the Loch Ness Monster exists since there is no evidence to the contrary.